This invention relates to camless internal combustion engines, and more particularly to providing compression release braking and other enhancements for such engines.
Most conventional internal combustion engines have rotating cams for causing the intake and exhaust valves in the engine cylinders to open and close at the appropriate times relative to reciprocation of the pistons in the cylinders. This type of engine construction has some limitations which have prompted consideration of alternative means for opening the intake and exhaust valves. For example, with cams for opening the cylinder valves it is difficult or impossible to adjust valve timing for different engine operating conditions (e.g., different engine speeds). The engine is therefore typically constructed so that it has optimum valve timing under one set of operating conditions (e.g., at a particular engine speed), thereby leaving valve timing somewhat suboptimal for other operating conditions (e.g., at other engine speeds). The amounts by which the valves open are also difficult or impossible to adjust for various operating conditions of engines with cams. Again, the engine is therefore typically constructed so that it has fixed valve openings which may be better for some engine operating conditions than for other engine operating conditions.
Because of the difficulty or impossibility of adjusting such parameters as valve timing and stroke in conventional internal combustion engines, various "camless" engines have been proposed. For example, Ule U.S. Pat. No. 4,009,695 purports to show engines in which the intake and exhaust valves are opened and closed by hydraulic actuators. The application of hydraulic fluid to these actuators is controlled by electrically operated hydraulic valves.
With many engines it is desirable to have both a positive power mode of operation (in which the engine produces power for such purposes as propelling an associated vehicle) and a braking mode of operation (in which the engine absorbs power for such purposes as slowing down an associated vehicle). As shown, for example, by Cummins U.S. Pat. No. 3,220,392, it is well known that a highly effective way of operating an engine in braking mode is to cut off the fuel supply to the engine and to then open the exhaust valves in the engine near top dead center of the compression strokes of the engine cylinders. This allows air that the engine has compressed in its cylinders to escape to the exhaust system of the engine before the engine can recover the work of compressing that air during the subsequent "power" strokes of the engine pistons. This type of engine braking is known as compression release engine braking.
It takes a great deal more force to open an exhaust valve to produce a compression release event during compression release engine braking than to open either an intake or exhaust valve during positive power mode operation of the engine. During positive power mode operation the intake valves typically open while the piston is moving away from the valves, thereby creating a low pressure condition in the engine cylinder. Thus the only real resistance to intake valve opening is the force of the intake valve return spring which normally holds the intake valve closed. Similarly, during positive power mode operation the exhaust valves typically open near the end of the power strokes of the associated piston after as much work as possible has been extracted from the combustion products in the cylinder. The piston is again moving away from the valves and the cylinder pressure against which the exhaust valves must be opened is again relatively low. (Once opened, the exhaust valves are typically held open throughout the subsequent exhaust stroke of the associated piston, but this only requires enough force to overcome the exhaust valve return spring force.)
During compression release engine braking, however, a much greater force is required to open the exhaust valves to produce a compression release event because such events are produced near top dead center of engine compression strokes when the gas pressure in the engine cylinders is close to a maximum. If the engine is a camless engine of the type in which the valves are opened by hydraulic actuators, very high pressure hydraulic fluid may have to be supplied to ensure that there is sufficient force available to open the exhaust valves during compression release engine braking. For example, the necessary hydraulic fluid pressure may be approximately 3,000-4,000 psi.
It is not a problem or even a disadvantage to provide high pressure hydraulic fluid during engine braking because the more energy the engine absorbs in this operating mode, the more braking it produces. During positive power mode operation, however, it is undesirable for the engine to be required to pump hydraulic fluid to such high pressures because this reduces the power available from the engine for useful, vehicle-propelling work.
In view of the foregoing, it is an object of this invention to improve the performance of camless engines in which the valves are opened hydraulically and which are capable of operating in a compression release engine braking mode as well as in a positive power mode.
It is a more particular object of this invention to avoid wasteful pumping of hydraulic fluid to very high pressures which are not needed during positive power mode operation of a camless engine, even though such high pressure hydraulic fluid may be needed during compression release engine braking mode operation of the engine.
Another characteristic of engine operation which it may be desirable to change when switching from positive power mode to compression release engine braking mode (or even when operating conditions change significantly within either of these two modes of engine operation) is the amount by which some or all of the valves in the engine cylinders open. For example, because exhaust valves open near top dead center of compression strokes of the engine cylinders during compression release engine braking, it may be desirable to reduce the exhaust valve stroke during engine braking to ensure that the exhaust valves do not hit the top of the engine pistons when the exhaust valves are opened. As another example, larger valve openings may be desirable during high speed positive power mode operation of the engine, while smaller valve openings may be preferable at lower speed positive power mode operation. Other changes in valve opening and closing trajectories may be desirable under various engine operating conditions. It may also be important to ensure that each valve returns to its seat without undue impact between the valve and seat.
It is therefore another object of this invention to facilitate modifying the stroke lengths and/or other characteristics of the trajectories of the cylinder valves in camless engines depending on various operating parameters of the engine.
It is still another object of this invention to facilitate reducing the impact between a closing valve and its seat in a camless internal combustion engine.